Gallium oxide (Ga2O3) is a transparent semiconductor that has a band gap as wide as 4.8-5.3 eV at room temperature and absorbs almost no visible light and ultraviolet light. It is accordingly a promising material for use in optical and electronic devices and transparent electronics operated particularly in a deep ultraviolet region. In recent years, photodetectors, light emitting diodes (LED), and transistors based on gallium oxide (Ga2O3) have been developed (refer to NPL 1).
Gallium oxide (Ga2O3) has five crystal structures of α, β, γ, δ, and ε, and generally the most stable structure is β-Ga2O3. However, having a β-gallic structure, β-Ga2O3 is not always preferred to be used in semiconductor devices, different from crystal systems generally used in electronic materials and the like. Growth of a β-Ga2O3 thin film requires a high substrate temperature and a high degree of vacuum, causing a problem of an increase in manufacturing costs. As described in NPL 2, in β-Ga2O3, even a high concentration (e.g., 1×1019/cm3 or more) dopant (Si) had to be annealed at high temperatures from 800° C. to 1100° C. after ion implantation to be used as a donor.
In contrast, having a crystal structure same as that of a sapphire substrate already sold for general purposes, α-Ga2O3 is accordingly preferred to be used in optical and electronic devices. It further has a band gap wider than that of β-Ga2O3, and thus is particularly useful for a power device and semiconductors device using α-Ga2O3 as the semiconductor are expected.
PTLs 1 and 2 discloses semiconductor devices that use β-Ga2O3 as a semiconductor and use an electrode, to obtain ohmic characteristics compatible with it, with two layers of a Ti layer and an Au layer, three layers of a Ti layer, an Al layer, and an Au layer, or four layers of a Ti layer, an Al layer, a Ni layer, and an Au layer.
PTL 3 discloses a semiconductor device that uses β-Ga2O3 as a semiconductor and uses any one of Au, Pt, or a laminate of Ni and Au as an electrode to obtain Schottky characteristics compatible with the semiconductor.
Unfortunately, application of the electrodes in PTLs 1 to 3 to a semiconductor device that uses α-Ga2O3 as a semiconductor causes a problem, such as not functioning as a Schottky electrode or an ohmic electrode, failing to attach the electrode to the film, and impairing semiconductor properties.